The Gom Jabbar, also known as "the high handed enemy", was a meta-cyanide poisoned needle that sat upon a thimble, and could thus be attached to a person's fingertip.
All the noble houses kept a Gom Jabbar of some sort, and used it to dispose of rivals and enemies under certain circumstances. One of the best known uses of the device occurred when the young Alia Atreides killed the Baron Vladimir Harkonnen during the Battle of Arrakeen, after declaring to him, "Meet the Atreides Gom Jabbar".
The Gom Jabbar Test of Humanity
- "Let us say I suggest you may be human. Steady! I warn you not to try jerking away. I am old, but my hand can drive this needle into your neck before you can escape me."
- ―Reverend Mother Gaius Mohiam while testing Paul Atreides with a Gom Jabbar[src]
The Bene Gesserit also made extensive use of the Gom Jabbar, especially when they tested the humanity of certain individuals. In such circumstances, the device was placed against the subject's neck and acted as a deterrent for them backing out of the test. Such a method was used by Reverend Mother Gaius Helen Mohiam against Paul Atreides before his arrival on Arrakis.
The gom jabbar test would be to determine whether an individual's awareness was stronger than their instincts. If their awareness of the gom jabbar's presence was strong enough, it would override their instincts to withdraw from the test, which usually involved great physical pain.
The origins of the device itself are not clear. Indeed, the origins of its name are also debatable. The first word, "Gom", could stem from the ancient acronym "GOM", which stood for "God's Own Medicine", and was often used in the context of a drug (particularly opium) or biological chemical on ancient Earth. The second part "Jabbar" comes clearly from the ancient Arabic word jabbār, meaning to use coercion to force something.